This relates to asphalt milling machines and methods, and controls for such machines. Asphalt milling machines are commonly used to prepare an existing asphalt road for repaving. Although it is possible simply to pave over an existing asphalt road with an additional layer of fresh asphalt paving material, this may not be desirable for several reasons. It will be appreciated, for example, that repeated paving of a road with additional asphalt may result in the road being raised to an undesirable elevation with respect to the surrounding terrain. This is of particular concern in those cases where the road is bounded by curbs and side walks, and where there are manhole openings in the road. With such a road, the addition of even a single layer of asphalt over the existing asphalt pavement may be unacceptable. Further, it is also not uncommon for the upper portions of an asphalt road surface to be in disrepair at the time that repaving is to begin. Additionally, the asphalt road surface may have also developed longitudinal waves over time. Clearly, simply adding a layer of asphalt over an irregular or deteriorating road surface may result in a paved surface that is not as smooth or as durable as desired. For these reasons, it is common to prepare an asphalt road for repaving by removing a portion of the existing asphalt from the road, producing a relatively smooth, sound surface for application of the new asphalt layer. This process has the additional advantage that it is possible to reuse the asphalt material that is removed from the road as a part of a subsequent repaving process.
Asphalt is removed from the top surface of the road that is to be repaved with an asphalt milling machine designed for the process. It will be appreciated that it is important to be able to control the depth of the milling process and the elevation of the resulting surface. A design surface, i.e., the desired surface that will be used as the base for the application of a new layer of asphalt, must be ground or shaved with some care, since the elevation of the surface will determine the elevation and orientation of the repaved road to a significant degree. Further, a roadbed that is ground to an elevation that is too low will require more than the desired amount of repaving material. On the other hand, a roadbed that is ground to an elevation that is too high will result either in a repaved surface that is too high or in a layer of repaved asphalt that is too thin. Additionally, since roads are typically milled by milling machines in a series of two or more parallel, abutting milling passes, it is important that the adjacent milled areas be ground to the same elevation.
Various controls have been used with milling machines, such as those that sense a string line positioned adjacent the milling path. Most milling machine controls have used a side plate that slides over an adjacent surface, with a sensor monitoring the vertical movement of the plate and the control using sensor output to control milling depth. Many milling machine controls use side plates on both sides of the machine for referencing the grinding to adjacent surfaces on each side. Other milling grade and slope control systems have used sonic tracers that measure the reference surface, string line, or curb elevation with pulses of sonic energy that are directed downward and then reflected back to the sensor. Still other systems have added a total station with a total station target on the milling machine, combined with a slope sensor, so that the movement of the machine can be monitored and controlled relative to the desired grade.
For relatively simple jobs, the common approach has been to bench the side plate to the cutting head and then lower the head until the desired depth is obtained. This can be done on both sides, or on just one side and use a cross-slope sensor to obtain the desired grade on the other side. If something other than a uniform depth of cut were needed to correct the road surface, a different approach was required. In such an instance, a surveyor would mark the road surface with indications of the desired depth at various points, and possibly the slope at those points, as well. This approach requires a machine operator to observe these markings, and to adjust the control point manually to produce a smooth transition between target depths. For more complex surfaces this is difficult to do, requiring constant adjustment by the operator. While three dimensional systems using total stations are capable of making the transitions automatically and providing a very precise result, there are other difficulties with their use. One such difficulty is that the line of sight of the total station can be blocked by traffic or other obstructions. In addition, a transition from one total station to another total station may be required if the working path of the asphalt grinding machine extends far enough.